- Email: [email protected]
Y multilayer thin films
Journal of Magnetism and Magnetic Materials 93 (1991) 449-451 North-Holland
449
Magnetic properties of F e / Y multilayer thin films R. 2;uberek, H. S z y m c z a k , R. K r i s h n a n ~ a n d T. M o r i s h i t a b Institute of" Physics of the Polish Academy of Sciences. 02-668 Warsaw, al. Lotnikow 32/46, Poland ~'Laboratoire de Magndtisme, CNRS, 92 195 Meudon cedex, France bNHK Science and Technical Research Laboratories, Setagaya, Tokyo 157, Japan
The results of saturation magnetization, magnetostriction constant and magneto-optical hysteresis loops in F e / Y modulated thin films prepared by evaporation are reported. All the m e a s u r e m e n t s have been performed at room temperature. It is demonstrated that the transformation of the Fe layers from amorphous structure to a bcc crystal has a strong influence on the magnetic properties of multilayers, In particular, magnetostriction constants are very sensitive to any structural transformation. Magnetic m e a s u r e m e n t s suggest an additional collinear magnetic structure of Fe layers and absence of any observable interdiffusion between the layers.
1. Introduction
The magnetic multilayer films have attracted considerable attention because their magnetic properties depend strongly on the modulation lengths of the layers and, what is more important, the possibility of inducing perpendicular anisotropy and so holds out the promise of new applications. In this p a p e r we investigate the relation between magnetic properties and structure in compositionally modulated F e / Y thin films. Of special interest have been structural sensitive magnetostriction measurements, particularly the dependence of the magnetostriction constant on the Fe layer thickness ( d w ) .
2. Experimental
The samples were evaporated by using a dual electron-beam source at a pressure of 2 × l 0 - 7 Torr or below. The deposition rate of 0.3 to 0.5 A / s was applied and each layer thickness was determined by quartz monitors. The glass substrates were mounted on the copper block cooled below 200K in order to reduce atomic diffusion at interfaces. The Y layer thickness (d v) was held o
constant at about 2 0 A for all the samples, and the Fe layer thickness was varied between 15 and 5 0 A [1]. Magnetization Ms, magneto-optical hysteresis loops and magnetostriction constant A, have been measured at room temperature as a function of the Fe layer thickness.
3. Results and discussion
Magnetization of the investigated multilayer films has been measured using a vibrating sample magnetometer. Fig. 1 shows the results of the saturation magnetization M s at room temperature as a function of the Fe layer thickness (dFe). It is seen that M s rapidly increases for the critical thickness dye = d c in the r a n g e from 20-25,~. For d w > d c the magnetization is close to the bulk value. It agrees with results obtained from high resolution T E M and Lorentz electron microscopy [1] that in F e / Y multilayers the Fe layer transforms from an amorphous structure (for d F e < d c) to a bcc crystal (for dye > dc). These films with dFo = d c show both an amorphous and crystalline behaviour. This nonhomogeneous state
0304-8853/91/$03.51/ © 1991- Elsevier Science Publishers B.V. (North-Holland)
450
R. Zuberek et aL / Fe / Y multilayer thin films 1600-
i
0 .=_
0
1200
E c~ 800 09
t
-5k6
400-
i
I
0
5kG
Fig. 3. Magneto-optical hysteresis loop for dF~ = 20~,. O 0
iiil,l,t
llrrr~
0
, Jt i,[r
1
tlt
t ii,lrlJ
2
Jt flit
3
lit
ifJl,t
t t [llltl,t
4
t j
6
5
~o [nm]
Fig. I. Magnetization as a function of the Fe layer thickness dF~. is clearly seen in the magnetostriction measurements. The magnetostriction constant As has been m e a s u r e d using the m e t h o d of strain m o d u l a t e d ferromagnetic resonance ( S M F M R ) [2, 3]. The results of m e a s u r e m e n t s are presented in fig. 2. The As has a positive value almost i n d e p e n d e n t
of dFe and a sharp peak for d F e = 20,~. For the film with dye = 2 0 A we observe two resonance fields in F M R and consequently two magnetostriction constants. Large value of k~ for d w = d c arises due to the strong dipolar fields existing in n o n h o m o g e n e o u s magnetic state. Recently, it has been shown that magnetostriction constants a s in N i / C and N i / A g multilayers d e p e n d linearly on the inverse thickness of magnetic layers "~s ~ 1 / d F e [4, 5]. In the case of F e / Y multilayer such d e p e n d e n c e is not observed. It has been shown that two mechanism are responsible for As ~ dmagn: of the magnetostrictive strains at the interface [6]; presence of the interdiffusion layers [7].
-localization
25 -
20-
Both m e n t i o n e d effects seem to be weak in the case of F e / Y multilayers. T h e absence of the interdiffused layers in our films has been confirmed by means of X-ray diffraction m e a s u r e m e n t s [8]. It is in contrast to results obtained in ref. [9]: in this work the interdiffused layers were responsible for asperomag-
15-
~I0-
Table 1 The saturation magnetization M, and Kerr rotation 0~<
-5
JIIIIIIIpFIIIIr
0
I
rrJTirr
2
JluulrrrJwlgllrwllr
5
Fe layer thickness dF~ (nm)
M~ (G)
OK (rain)
1.5 2.0 2.5
100 785 1 280
24.2 33.7 49.0
JJllIllrlrwwuulllr
4
5
iv. [nm] Fig. 2. Magnetostrictionconstantvs. Fe layer thickness.
6
R. Zuberek et al. / Fe / Y multilayer thin films
netic b e h a v i o u r o f the F e / Y multilayers. It s e e m s t h a t interdiffusion o b s e r v e d in [9] is d u e to high t e m p e r a t u r e o f s u b s t r a t e s d u r i n g t h e film growing processes. M o r e o v e r , m a g n e t o - o p t i c a l m e a s u r e m e n t d i d not confirm a s p e r o m a g n e t i c p r o p e r t i e s of i n v e s t i g a t e d multilayers. M a g n e t o - o p t i c a l hysteresis l o o p s have b e e n m e a s u r e d with H e - N e light using the m a g n e t o optical K e r r effect. Fig. 3 shows a hysteresis l o o p o b t a i n e d at r o o m t e m p e r a t u r e for the film with d~e = 2 0 A . T h e form of the loop is c h a r a c t e r i s t i c of f e r r o m a g n e t i c a l l y o r d e r e d s a m p l e s r a t h e r t h a n a s p e r o m a g n e t i c . F o r films o f d F e > 25.~ we cannot o b t a i n the s a t u r a t i o n m a g n e t i z a t i o n in configuration p e r p e n d i c u l a r to the film. A s the dye i n c r e a s e s the K e r r r o t a t i o n OK i n c r e a s e s too, a n d the l o o p b e c o m e s h a r d e r to m a g n e t i z e p e r p e n d i c u l a r to t h e film p l a n e . It has b e e n shown (see T a b l e 1) that the K e r r r o t a t i o n OK is p r o p o r t i o n a l to the m a g n e t i z a t i o n M s a n d i n c r e a s e s with d w .
4. Conclusion In this p a p e r we have d e m o n s t r a t e d that the t r a n s f o r m a t i o n of the F e layers from a m o r p h o u s s t r u c t u r e to a bcc crystal has a strong influence on the m a g n e t i c p r o p e r t i e s o f the multilayers. In p a r t i c u l a r , m a g n e t o s t r i c t i o n c o n s t a n t s are very sensitive to any s t r u c t u r a l t r a n s f o r m a t i o n . M a g netic measurements s u g g e s t an a d d i t i o n a l
451
c o l l i n e a r m a g n e t i c s t r u c t u r e o f F e layers a n d absence o f any o b s e r v a b l e interdiffused layer.
Acknowledgement This w o r k was partly s u p p o r t e d by the C P B P p r o g r a m u n d e r C o n t r a c t 01,04.
References [1] T. Morishita, Y. Togami, K. Tsushima and S, Tsukahara, Proc. Intern. Syrup. on Physics of Magnetic Materials, Sendai, 8-11 April 1987, eds. M. Takahashi, S. Maekawa, Y. Gondo and H. Nose (World Scientific, Singapore, 1987) p. 295. [2] J. Wosik, K. Nesteruk, W. Zbieranowski and A. Sienkiewicz, J. Phys. E 11 (1978) 1200. [3] J.C.M. Henning and J.H. den Boef, Appl. Phys. 16 (1978) 353. [4] H. Szymczak, R. J~uberek, R. Krishnan, M. Tessier, K.B. Youn and C. Sella, ICMFS-12 Conf., Le Creusot (1988) abstract TH3-03. [5] R. Zuberek, H. Szymczak, R. Krishnan and M. Tessier, J. de Phys. 49 (1988) C8-1761. [6] H. Szymczak, R. Zuberek, R. Krishnan and M. Tessier, Intermag'90, 17-20 April 1990 Brighton, UK, paper HC09. [7] F.W.A. Dirne and C.J.M. Denissen, J. Magn. Magn. Mat. 78 (1989) 122. [8] R. Sato, M. Kajiura, T. Morishita and K. Tsushima, Proc. Intern. Symp. on Physics of Magnetic Materials, Sendai, 8-11 April 1987, eds. M. Takahashi, S. Maekawa, Y. Gondo and H. Nose (World Scientific, Singapore, 1987) p. 299. [9] F. Badia, C. Ferrater, A. Lousa, B. Martinez, A. Labarta and J. Teiada, 34th 3-MMM Conf., Boston, Massachusetts, 28 November-I December 1989, USA, paper FP-10.
449
Magnetic properties of F e / Y multilayer thin films R. 2;uberek, H. S z y m c z a k , R. K r i s h n a n ~ a n d T. M o r i s h i t a b Institute of" Physics of the Polish Academy of Sciences. 02-668 Warsaw, al. Lotnikow 32/46, Poland ~'Laboratoire de Magndtisme, CNRS, 92 195 Meudon cedex, France bNHK Science and Technical Research Laboratories, Setagaya, Tokyo 157, Japan
The results of saturation magnetization, magnetostriction constant and magneto-optical hysteresis loops in F e / Y modulated thin films prepared by evaporation are reported. All the m e a s u r e m e n t s have been performed at room temperature. It is demonstrated that the transformation of the Fe layers from amorphous structure to a bcc crystal has a strong influence on the magnetic properties of multilayers, In particular, magnetostriction constants are very sensitive to any structural transformation. Magnetic m e a s u r e m e n t s suggest an additional collinear magnetic structure of Fe layers and absence of any observable interdiffusion between the layers.
1. Introduction
The magnetic multilayer films have attracted considerable attention because their magnetic properties depend strongly on the modulation lengths of the layers and, what is more important, the possibility of inducing perpendicular anisotropy and so holds out the promise of new applications. In this p a p e r we investigate the relation between magnetic properties and structure in compositionally modulated F e / Y thin films. Of special interest have been structural sensitive magnetostriction measurements, particularly the dependence of the magnetostriction constant on the Fe layer thickness ( d w ) .
2. Experimental
The samples were evaporated by using a dual electron-beam source at a pressure of 2 × l 0 - 7 Torr or below. The deposition rate of 0.3 to 0.5 A / s was applied and each layer thickness was determined by quartz monitors. The glass substrates were mounted on the copper block cooled below 200K in order to reduce atomic diffusion at interfaces. The Y layer thickness (d v) was held o
constant at about 2 0 A for all the samples, and the Fe layer thickness was varied between 15 and 5 0 A [1]. Magnetization Ms, magneto-optical hysteresis loops and magnetostriction constant A, have been measured at room temperature as a function of the Fe layer thickness.
3. Results and discussion
Magnetization of the investigated multilayer films has been measured using a vibrating sample magnetometer. Fig. 1 shows the results of the saturation magnetization M s at room temperature as a function of the Fe layer thickness (dFe). It is seen that M s rapidly increases for the critical thickness dye = d c in the r a n g e from 20-25,~. For d w > d c the magnetization is close to the bulk value. It agrees with results obtained from high resolution T E M and Lorentz electron microscopy [1] that in F e / Y multilayers the Fe layer transforms from an amorphous structure (for d F e < d c) to a bcc crystal (for dye > dc). These films with dFo = d c show both an amorphous and crystalline behaviour. This nonhomogeneous state
0304-8853/91/$03.51/ © 1991- Elsevier Science Publishers B.V. (North-Holland)
450
R. Zuberek et aL / Fe / Y multilayer thin films 1600-
i
0 .=_
0
1200
E c~ 800 09
t
-5k6
400-
i
I
0
5kG
Fig. 3. Magneto-optical hysteresis loop for dF~ = 20~,. O 0
iiil,l,t
llrrr~
0
, Jt i,[r
1
tlt
t ii,lrlJ
2
Jt flit
3
lit
ifJl,t
t t [llltl,t
4
t j
6
5
~o [nm]
Fig. I. Magnetization as a function of the Fe layer thickness dF~. is clearly seen in the magnetostriction measurements. The magnetostriction constant As has been m e a s u r e d using the m e t h o d of strain m o d u l a t e d ferromagnetic resonance ( S M F M R ) [2, 3]. The results of m e a s u r e m e n t s are presented in fig. 2. The As has a positive value almost i n d e p e n d e n t
of dFe and a sharp peak for d F e = 20,~. For the film with dye = 2 0 A we observe two resonance fields in F M R and consequently two magnetostriction constants. Large value of k~ for d w = d c arises due to the strong dipolar fields existing in n o n h o m o g e n e o u s magnetic state. Recently, it has been shown that magnetostriction constants a s in N i / C and N i / A g multilayers d e p e n d linearly on the inverse thickness of magnetic layers "~s ~ 1 / d F e [4, 5]. In the case of F e / Y multilayer such d e p e n d e n c e is not observed. It has been shown that two mechanism are responsible for As ~ dmagn: of the magnetostrictive strains at the interface [6]; presence of the interdiffusion layers [7].
-localization
25 -
20-
Both m e n t i o n e d effects seem to be weak in the case of F e / Y multilayers. T h e absence of the interdiffused layers in our films has been confirmed by means of X-ray diffraction m e a s u r e m e n t s [8]. It is in contrast to results obtained in ref. [9]: in this work the interdiffused layers were responsible for asperomag-
15-
~I0-
Table 1 The saturation magnetization M, and Kerr rotation 0~<
-5
JIIIIIIIpFIIIIr
0
I
rrJTirr
2
JluulrrrJwlgllrwllr
5
Fe layer thickness dF~ (nm)
M~ (G)
OK (rain)
1.5 2.0 2.5
100 785 1 280
24.2 33.7 49.0
JJllIllrlrwwuulllr
4
5
iv. [nm] Fig. 2. Magnetostrictionconstantvs. Fe layer thickness.
6
R. Zuberek et al. / Fe / Y multilayer thin films
netic b e h a v i o u r o f the F e / Y multilayers. It s e e m s t h a t interdiffusion o b s e r v e d in [9] is d u e to high t e m p e r a t u r e o f s u b s t r a t e s d u r i n g t h e film growing processes. M o r e o v e r , m a g n e t o - o p t i c a l m e a s u r e m e n t d i d not confirm a s p e r o m a g n e t i c p r o p e r t i e s of i n v e s t i g a t e d multilayers. M a g n e t o - o p t i c a l hysteresis l o o p s have b e e n m e a s u r e d with H e - N e light using the m a g n e t o optical K e r r effect. Fig. 3 shows a hysteresis l o o p o b t a i n e d at r o o m t e m p e r a t u r e for the film with d~e = 2 0 A . T h e form of the loop is c h a r a c t e r i s t i c of f e r r o m a g n e t i c a l l y o r d e r e d s a m p l e s r a t h e r t h a n a s p e r o m a g n e t i c . F o r films o f d F e > 25.~ we cannot o b t a i n the s a t u r a t i o n m a g n e t i z a t i o n in configuration p e r p e n d i c u l a r to the film. A s the dye i n c r e a s e s the K e r r r o t a t i o n OK i n c r e a s e s too, a n d the l o o p b e c o m e s h a r d e r to m a g n e t i z e p e r p e n d i c u l a r to t h e film p l a n e . It has b e e n shown (see T a b l e 1) that the K e r r r o t a t i o n OK is p r o p o r t i o n a l to the m a g n e t i z a t i o n M s a n d i n c r e a s e s with d w .
4. Conclusion In this p a p e r we have d e m o n s t r a t e d that the t r a n s f o r m a t i o n of the F e layers from a m o r p h o u s s t r u c t u r e to a bcc crystal has a strong influence on the m a g n e t i c p r o p e r t i e s o f the multilayers. In p a r t i c u l a r , m a g n e t o s t r i c t i o n c o n s t a n t s are very sensitive to any s t r u c t u r a l t r a n s f o r m a t i o n . M a g netic measurements s u g g e s t an a d d i t i o n a l
451
c o l l i n e a r m a g n e t i c s t r u c t u r e o f F e layers a n d absence o f any o b s e r v a b l e interdiffused layer.
Acknowledgement This w o r k was partly s u p p o r t e d by the C P B P p r o g r a m u n d e r C o n t r a c t 01,04.
References [1] T. Morishita, Y. Togami, K. Tsushima and S, Tsukahara, Proc. Intern. Syrup. on Physics of Magnetic Materials, Sendai, 8-11 April 1987, eds. M. Takahashi, S. Maekawa, Y. Gondo and H. Nose (World Scientific, Singapore, 1987) p. 295. [2] J. Wosik, K. Nesteruk, W. Zbieranowski and A. Sienkiewicz, J. Phys. E 11 (1978) 1200. [3] J.C.M. Henning and J.H. den Boef, Appl. Phys. 16 (1978) 353. [4] H. Szymczak, R. J~uberek, R. Krishnan, M. Tessier, K.B. Youn and C. Sella, ICMFS-12 Conf., Le Creusot (1988) abstract TH3-03. [5] R. Zuberek, H. Szymczak, R. Krishnan and M. Tessier, J. de Phys. 49 (1988) C8-1761. [6] H. Szymczak, R. Zuberek, R. Krishnan and M. Tessier, Intermag'90, 17-20 April 1990 Brighton, UK, paper HC09. [7] F.W.A. Dirne and C.J.M. Denissen, J. Magn. Magn. Mat. 78 (1989) 122. [8] R. Sato, M. Kajiura, T. Morishita and K. Tsushima, Proc. Intern. Symp. on Physics of Magnetic Materials, Sendai, 8-11 April 1987, eds. M. Takahashi, S. Maekawa, Y. Gondo and H. Nose (World Scientific, Singapore, 1987) p. 299. [9] F. Badia, C. Ferrater, A. Lousa, B. Martinez, A. Labarta and J. Teiada, 34th 3-MMM Conf., Boston, Massachusetts, 28 November-I December 1989, USA, paper FP-10.